1. Field of the Invention
The present invention relates to an oxidative decontamination reagent for removal of the dense radioactive oxide layer on the metal surface and a method for oxidative decontamination using the same. More particularly, the invention relates to an oxidative decontamination reagent for removal of the dense radioactive oxide layer on the metal surface including an oxidizing agent, a metal ion, and an inorganic acid, and a method for oxidative decontamination using the same.
2. Description of the Related Art
The major metal parts composing the nuclear power plant system can be corroded by the steam or coolant circulating the nuclear power plant, so that a dense metal oxide can be formed on the metal surface as a minor corrosion product. The metal oxide is contaminated by a radioactive substance, resulting in the accumulation of such radioactive substance in the nuclear power plant system.
Moreover, what makes us more worried is the incidence of radiation exposure to workers in and around the nuclear power plant (for example, the power plant operators and maintenance people), which is inevitable once a radioactive substance is accumulated in the plant system to emit radiation. The metal oxide contains radionuclide which increases the level of radiation around the nuclear power plant system.
As a part of the effort to reduce radiation exposure to the nuclear power plant workers, studies have been focused world-widely on the elimination of radioactivity sticking on the steam generator water chamber and coolant pump. That is, radioactivity has been the major concern and target to reduce radiation exposure to the workers. Up to date, a variety of methods have been tried to eliminate radioactivity and a technique to eliminate metal oxide contaminated by radioactivity is under-development.
The chemical decontamination method is the technique for the decontamination of metal parts of the nuclear power plant system. To perform the optimum decontamination, the radioactive contaminated objectives have to be eliminated along with the corroded oxide layer without damaging metal parts themselves. It is also necessary to minimize the secondary waste produced during the decontamination process.
The conventional methods for the decontamination of the nuclear power plant system depend on nitric permanganate (NP) decontamination reagents and alkaline permanganate (AP) decontamination reagents.
However, the conventional AP decontamination reagent has the disadvantage of massive production of secondary waste even though corrosion resistance on metal parts is excellent. And, NP decontamination reagent has the disadvantage of high corrosion rate of metal parts even though it has the advantage of low production of secondary waste.
The material of the steam generator heat exchanger tube of pressurized water reactor is made of inconel nickel alloy. Residual stress is observed in the part of U-band or expanded tube sheet of such heat pipes, suggesting that the incidence of local corrosion such as stress corrosion cracking (SCC) and intergranular corrosing attack (ICA) is high even during the normal operation of the plant. When those parts showing residual stress are exposed on the oxidative decontamination reagent, the reagent can be permeated through vacancy or cracks in the corroded oxide layer to cause corrosion of the metal parts. When the oxide layer is decomposed during oxidative decontamination, the metal parts are exposed and local corrosion thereon progresses. So, it is requested to develop a method to prevent such local corrosion.
As an example of the conventional methods, Japanese Patent No. 4083607 describes a method and device for chemical decontamination of radioactivity. Particularly, the target substance contaminated by radioactivity is dipped in a decontamination solution containing acid, so that the surface of the target substance is decomposed by lowering electric potential of the target substance up to the level of corroded area therein and then the metal ions in the decontamination solution are eliminated by using cation exchange resin. However, the said chemical decontamination method has a problem of difficulty in prevention of metal part corrosion.
Korean Patent Publication No. 10-2008-0056846 describes a method for diluted chemical decontamination of NP reactor coolant pump internal contaminated by radioactive substance. Particularly, the invention provides a method for decontamination performed in a specific procedure, chemical, concentration, operation time, and operation temperature. However, The inconvenience of this method is that all the conditions for the decontamination have to be altered according to the materials and internals of the nuclear power plant system and the prevention of corrosion of metal parts is still a problem.
Korean Patent Publication No. 10-2001-0108013 describes a method for nuclear power plant decontamination. Particularly, this method is to reduce pollutants from metal parts by reducing adhesiveness of oxide layer on the metal surface by introducing a low concentration of zinc ion into primary coolant of the running boiling water reactor. This method is rather the way to reduce pollutants by zinc injection to the primary coolant of the running reactor than the technique related to decontamination.
In the course of the study to establish a method for oxidative decontamination on metal parts of the nuclear power plant system, the present inventors were succeeded in regulating electric potential of the metal parts to be passive state by adding the decontamination reagent composed of an oxidizing agent, an inorganic acid, and additionally a metal ion, and further developed an oxidative decontamination reagent (MONAP, MOdified Nitric Acid Permanganate) that is characterized by less chance of local corrosion and less secondary waste as well, leading to the completion of the present invention.